Had Jack Crompton, who has died aged 91, served in the Royal Air Force as a rear gunner as he had intended, Manchester United might have been denied a reliable goalkeeper who served them efficiently in 212 appearances, including the 1948 FA Cup final. "Tail-end Charlies", squatting in the rear turret of bombers, had a particularly perilous existence. Crompton's, and United's, fortune was that a football injury rendered him unfit for military service.

He was born and brought up in Hulme, Manchester, the youngest of the six children of James and Eliza Crompton. After the second world war broke out he shovelled coal for locomotives before deciding to apply for the RAF. He suffered his injury playing for Newton Heath Loco and later appeared for the non-league side Goslings. There he was spotted by the Manchester United scout Louis Rocca.

Crompton played one game as an amateur for Manchester City, but the chance to earn a living with United was irresistible. In the war years, when "guest" players proliferated, he also played for Stockport County, who paid him better than United. A stickler for fitness, Crompton was a regular at the YMCA, where he would later, as a trainer, take United's players.

He made his debut in goal for United on 5 August 1946 and the highlight of his career came at Wembley on 24 April 1948 with the FA Cup final against Blackpool and their formidable winger Stanley Matthews. Blackpool took the lead from a penalty taken by Eddie Shimwell, but manager Matt Busby's words – "keep on playing football" – calmed and encouraged United. Just before half time, Crompton made the save that arguably enabled United to win the match. They were 2-1 behind when Walter Rickett, the Blackpool winger, shot, but Crompton reached the ball and turned it for a corner. United eventually won 4-2, bringing Busby his first trophy as the club's manager.

Crompton had almost missed the match due to an abscess at the base of his spine. At Ancoats hospital in Manchester – where, after his previous injury, a doctor had advised him to sell his football boots – Crompton underwent an operation days before the final.

In the 1948-49 season, he missed just one game in the First Division, but the following season was ruled out for weeks with a broken wrist. Though he felt capable of playing on, he eventually lost his place to Reg Allen – who was the side's regular keeper when they won the league in 1952. In the 1952-53 season Crompton made 25 league appearances and in the following season 15 more. Over the next two seasons he took part in a mere half-dozen league matches. Having played loyally and happily for United's reserves, he retired in 1955 and, with Busby's encouragement, took the trainer's job at Luton.

In 1958, he returned to Wembley with United – this time in a backroom role – for another FA Cup final. The team was urgently cobbled together after the horrific Munich air crash in February that year, which resulted in the deaths of eight United players. Nobby Stiles later wrote that "bringing Jack back to the club was a masterstroke. He was just what was required following the horror which had struck the club in Bavaria. He was the ideal man to help bring some much-needed stability to the dressing room and the training ground."

In 1962, Luton invited Crompton to become their manager, but after seven days in the job, and much soul-searching, he elected to stay at Old Trafford. He again played a key role in the backroom, this time including United's 4-1 victory against Benfica in the European Cup final at Wembley in 1968.

Crompton subsequently managed Barrow in the Fourth Division but could not prevent them from having to apply for re-election to the league (they were turned down). He also had spells as a trainer with Bury and, under Bobby Charlton, at Preston.

He returned to Old Trafford to look after the reserves for seven years from 1974. In 1981, after the departure of Dave Sexton and before the appointment of Ron Atkinson, he was made temporary manager of United. As such, he took a severely depleted squad on a surprisingly successful if nerve-racking tour of east Asia.

Crompton later took a job coaching with the Sports Council at Manchester City's Platt Lane training centre. He remained a familiar and much-loved figure at Old Trafford as well as in non-league football in Manchester. In 2008, at the age of 86, he published an engaging autobiography, From Goal Line to Touchline.

Crompton is survived by his second wife, Sheila, and by his daughter, Joan, from his first marriage, to Teresa.

• John Crompton, footballer and coach, born 18 December 1921; died 4 July 2013